Almost half of pharmaceuticals on the market target G-protein coupled receptors (GPCRs). A large portion of GPCRs signal through Gq proteins, which activate phospholipase C and result in flux of Ca2+ ions through calcium channels. Initial screens of candidate GPCR agonists or antagonists are routinely performed by fluorescence imaging of a small molecule Ca2+ signaling dye. The target (Gq-signaling) receptor is over-expressed in a cell line and the cells are incubated with a Ca2+ dye. Addition of an agonist increases, and an antagonist decreases, dye fluorescence. Titration of this effect, and/or competition with endogenous agonist, reveals an apparent Ki of inhibition of the receptor. Another large (and growing) class of drug targets is ion channels themselves (many of which conduct Ca2+). Imaging [Ca2+] is a direct measurement of the activity of Gq GPCRs and cation channels, and visualization of [Ca2+] flux is useful for assaying the effect of potential drugs on such protein targets.
Calcium is a universal second messenger regulating essential cellular signaling events in a broad range of tissues and organisms. In neurons, action potentials (APs) trigger large and rapid changes in cytoplasmic free calcium. Similarly, activation of synaptic glutamate receptors during synaptic transmission produces [Ca2+] transients in dendritic spines. Calcium imaging using synthetic calcium indicators has been used to measure neuronal spiking and synaptic input across populations of neurons in vitro and in vivo. However, synthetic indicators are difficult to target to specific cell types or sub-cellular locations. The loading procedures are invasive and damaging to neural tissue, precluding repeated, chronic in vivo measurements.